We are currently hanging out at our old farm house which has an outside combo heat/air unit; the heat part is propane. The filter is currently inside the unit, which is outside; our return air is in the floor inside. My brother has the same situation at his house, but he filters his with a thich, washable filter at his return air inside. Could I do that too? It would be much easier, especially when it was cold and snowy outside.

If the inside air is recirculated over and over like my system you need a filter inside. My air originates inside and never goes outside. It's needed to capture the unbelievable amount of dust our old carpet produces and it would stop up the inside evaporator in short order. I can see where the outside filter would be needed to capture the dust and other things you see in the air outside if it goes straight through the heat exchanger. You need to keep the duct work clean with a filter before the air enters it.

If the inside air is recirculated over and over like my system you need a filter inside. My air originates inside and never goes outside. It's needed to capture the unbelievable amount of dust our old carpet produces and it would stop up the inside evaporator in short order. I can see where the outside filter would be needed to capture the dust and other things you see in the air outside if it goes straight through the heat exchanger. You need to keep the duct work clean with a filter before the air enters it.

So, would a filter in both places be good? My brother said his installer said not to filter at both places because it would strain the unit, but that could be a matter of opinion.

It does not hurt to filter it inside, but you do not want to restrict the air flow
so you might have to take out the outside filter. If you are using a thin air filter then you might want to go to a heating wholesale house and see if you can find a spray filter coat so you can spray the filter and it will catch the dust much better. I don't know what size your unit is and how much CFM of air it is moving, but make sure the filter inside is as big as the one outside. You should have one sq. foot of filter for every 300 CFM of air being moved Later Paul